1. Field of the Invention:
This invention relates to a lens barrel attached to and used for an optical instrument such as a camera.
2. Description of the Related Art:
The current progress in electronic devices used in single-lens reflex cameras has brought about a remarkable improvement in function and utility of this type of cameras particularly when the cameras are used with lens units which are designed as original parts of the respective cameras. In general, electronic devices mounted in a camera, designed as an integrated circuit (referred to as IC), has a certain size. It is not easy to find a space for mounting such an IC in a restricted area of the camera, particularly in a lens barrel, and one solution to this problem is to increase the outside diameter of the lens barrel. Furthermore, telephoto lens barrels and zoom lens barrels of large magnification require that ICs incorporated in such lens barrels have highly complicated control circuits and various types of information in the form of electrical signals corresponding to the lens specifications, with the result that the sizes of the lens barrels are inevitably increased. On the other hand, lens barrels are required to meet, about all, a requirement for reduction in the size. Apparently, such a requirement is incompatible with the demand for installation of large ICs. Thus, it has been generally difficult to electronically represent a variety of lens specifications, and realization of high degree of electronical control has been almost impossible.
FIG. 6 shows, by way of example, a zoom lens barrel (telephoto zoom lens) incorporating an IC.
The lens barrel comprises a body member supporting a lens group and has a mount 1 which is known per se and which is provided with a claw la for coupling to a camera body. The lens barrel further has a fixed tube 2 which is integral with the mount 1 and which is provided in the peripheral surface thereof with a cam groove 2a, and a movable tube 3 which fits on the outer peripheral surface of the fixed tube 2 for axial sliding movement thereon and which is provided in the peripheral surface thereof with a cam groove 3a. The inner peripheral surface of the front end portion of the movable tube 3 is provided with a screw thread 3b. The screw thread 3b engages with a screw thread 4a formed in the outer peripheral surface of a first lens holder tube 4 which is supported by the movable tube 3 to be axially movable and rotatable. Gear teeth 4b are formed on the outer peripheral surface of the front end portion of the first lens holder tube 4. A motor 7 is fixed to the outer peripheral surface of a front end portion of the movable tube 3. The motor 7 has an output shaft carrying a pinion 6 which is slidable in the axial direction and which is always held in engagement with the gear teeth 4b. Supply of electrical power and control signals to the motor 7 is conducted through a flexible printed circuit board 8 which extends along the outer peripheral surfaces of the movable tube 3 and the fixed tube 2 in the axial direction. The flexible printed circuit board 8 is connected at its rear end to a connecting terminal 11 which is provided on the rear end surface of the mount 1.
A second lens holder tube 5 fits in a front end portion of the fixed tube 2 for movement both in the axial direction and rotational direction. A pin 5a projecting from the outer peripheral surface of the second lens holder tube 5 is slidably received in the cam groove 2a of the fixed tube 2 and in the cam groove 3a of the movable tube 3. A first lens I held by the first lens holder tube 4 and a second lens II held by the second lens holder tube 5 are adapted to be moved axially in response to an axial movement of the movable tube 3 caused by manual effort of the photographer, whereby zooming control is effected. The movable tube 3 is provided with a diaphragm unit 13 which is known per se.
The fixed tube 2 serves also as a third lens holder tube. More specifically, a third lens III is fixed in a lens holding frame 2b which projects radially inwardly from the rear end of the fixed tube 2. An annular disc-shaped circuit board 9, which carries a circuit element (large-sized IC) 12 for controlling the motor 7, the diaphragm unit 13, etc., is fixed to an annular end plate portion 2c which is formed in an annular shape along the outer periphery of the lens holding frame 2b, by means of a screw 10. Circuits on the circuit board 9 also are electrically connected to the connecting terminal 11 on the mount 1.
The circuit board 9 provided on the zoom lens barrel known per se is located at the same position as the third lens III in the direction of the optical axis. This essentially requires that a central aperture 9a of the circuit board 9 has a considerably large diameter and, hence, the effective area of the annular circuit board 9 available for carrying the electronic elements is undesirably limited. Therefore, the circuit board 9 can carry only a small IC chip which can scarcely include a circuit for controlling the motor 7 and the circuit for controlling the diaphragm unit 13. Thus, the circuit board 9 cannot carry a large-sized IC chip which contains not only the circuits for controlling the motor 7 and the diaphragm unit 13 but also data processing circuits for processing data concerning a zooming operation and control circuits for actually executing the zooming operation. In consequence, the camera which makes use of interchangeable lens units of the type described could have only limited specifications, thus failing to meet the demand for an automatic photographing operation for realizing a high level of photographing technique.
The annular disc-shaped circuit board arranged in the vicinity of the mount of the lens barrel is shown in the drawings attached to the specification of U.S. Patent Application Ser. No. 134,976 filed on Dec. 18, 1987, now U.S. Pat. No. 4,804,331.
On the other hand, various detrimental light components such as light component reflected by the inner surface of the lens barrel and light component reflected by the glass surface inevitably exist in the lens barrel. These detrimental light components do not contribute to the formation of image but, rather, impedes the image formation seriously. It is known to provide a light-shielding plate (fixed diaphragm) in the vicinity of the mount of the lens barrel in order to obtain a good image forming performance by excluding the portion of the light beam which does not make contribution to the image formation in a rectangular image forming region on the film. The light-shielding plate has a substantially rectangular aperture which passes only the portion of the light beam which contributes to image formation while excluding the detrimental light components, so that any unfavorable effect on the image formation which may otherwise be caused by the detrimental light components is avoided.